STANFORD L INEAR ACCELERATOR CENTER

Fall 2001, Vol. 31, No. 3

Guest EditorMICHAEL RIORDAN

EditorsRENE DONALDSON, BILL KIRK

Contributing EditorsGORDON FRASER

JUDY JACKSON, AKIHIRO MAKIMICHAEL RIORDAN, PEDRO WALOSCHEK

Editorial Advisory BoardPATRICIA BURCHAT, DAVID BURKE

LANCE DIXON, EDWARD HARTOUNIABRAHAM SEIDEN, GEORGE SMOOT

HERMAN WINICK

IllustrationsTERRY ANDERSON

DistributionCRYSTAL TILGHMAN

A PERIODICAL OF PARTICLE PHYSICS

CONTENTS

FALL 2001 VOL. 31, NUMBER 3

The Beam Line is published quarterly by the Stanford Linear Accelerator Center, Box 4349,Stanford, CA 94309. Telephone: (650) 926-2585.EMAIL: beamline@slac.stanford.eduFAX: (650) 926-4500

Issues of the Beam Line are accessible electroni-cally on the World Wide Web at http://www.slac.stanford.edu/pubs/beamline. SLAC is operated byStanford University under contract with the U.S.Department of Energy. The opinions of the authorsdo not necessarily reflect the policies of theStanford Linear Accelerator Center.

Cover: The Sudbury Neutrino Observatory detectsneutrinos from the sun. This interior view frombeneath the detector shows the acrylic vesselcontaining 1000 tons of heavy water, surrounded by photomultiplier tubes. (Courtesy SNOCollaboration)

Printed on recycled paper

2 FOREWORDDavid O. Caldwell

FEATURES

4 PAULI’S GHOSTA seventy-year saga of the conceptionand discovery of neutrinos.Michael Riordan

15 MINING SUNSHINEThe first results from the SudburyNeutrino Observatory revealthe “missing” solar neutrinos.Joshua Klein

24 NEUTRINO EXPERIMENTSAT FERMILABNeutrino physics is thrivingin the Nation’s heartland.Paul Nienaber

32 THE ENIGMATIC WORLDOF NEUTRINOSTrying to discern the patterns ofneutrino masses and mixing.Boris Kayser

42 THE K2K NEUTRINOEXPERIMENTThe world’s first long-baselineneutrino experiment is beginningto produce results.Koichiro Nishikawa & Jeffrey Wilkes

50 WHATEVER HAPPENEDTO HOT DARK MATTER?Neutrinos may have had asignificant impact on thestructure of the Universe.Joel R. Primack

58 CONTRIBUTORS

DATES TO REMEMBER

24 FALL 2001

ARTICLE PHYSICS EXPERIMENTS oftenget lumped under the general heading of “bigscience.” Particle accelerators are gargantuan,detectors are huge and complex, and today’smulti-institutional collaborations generatesocial dynamics whose unraveling would

baffle the most sophisticated supercomputer. Neutrinoexperiments are no exception to this observation; thedetectors come in three sizes: large, extra-large, andpositively Bunyanesque. Yet, like the graceful hippopotamiin Walt Disney’s Fantasia, a “little” neutrino experimentcan on occasion dance briskly in, do some beautiful tour-de-force physics, and take a bow—all for a relatively modestticket price. Two premier danseurs of this sort on theFermilab stage are the DONUT and BooNE experiments.DONUT (Direct Observation of the Nu-Tau) has completedits run and is now accepting bravos for being the firstexperiment to detect the tau neutrino. BooNE (BoosterNeutrino Experiment) is waiting in the wings, warming upto delve more deeply into the peculiar pas de deux ofneutrino oscillations.

According to the Standard Model, our current bestworking theory of elementary particles and fundamentalforces, a neutrino can interact and change into its electricallycharged lepton partner by emitting or absorbing the chargedcarrier of the weak force, the W boson. This type of exchangeis called a “charged-current” interaction (to distinguish it

Neutrino Experiments at by PAUL NIENABER

PNeutrino physics

is thriving

in the

Nation’s

heartland.

BEAM LINE 25

Fermilab

from the exchange of a neutral Z boson). The weak force also obeys a two-fold lepton-number conservation rule: both the number of leptons and theirkind remains unchanged. Interacting electron neutrinos yield only electrons,muon neutrinos only muons, tau neutrinos only taus. Neutrinos and theircharged partners can switch back and forth within a Standard Model genera-tion, but they cannot jump between generations. This conservation rulemakes the charged-current interaction a useful “generation tagger”: if yousee a muon zipping through your neutrino detector, and you can ascertainthat it came directly from a neutrino interaction, you can be sure it wasproduced by a muon neutrino.

Because neutrinos are immune to the strong and electromagneticinteractions, they provide a unique “weak force scalpel” for investigating theproperties of particles. Electron neutrinos are difficult to concentrate into abeam whose direction and energy can be varied, so muon neutrinos have be-come the scalpel of choice. The recipe is fairly straightforward: take a protonbeam, smack it into a block of material, magnetically focus and direct thecharged pions and kaons spewing forth from the collision, allow them to de-cay into muons and muon neutrinos, filter out the charged decay productsand any other unwanted debris, and voila! You have a beam of muonneutrinos.

But because neutrinos interact so weakly, the chances of any singleone interacting in a detector are minuscule. The probability of witnessingan interaction depends on the number of neutrinos, their energy, and thenumber of other particles available for them to hit. Getting a reasonablenumber of events therefore requires lots of neutrinos, lots of energy, and lots and lots of detector material—hence the Brobdingnagian scale of neutrinoexperiments.

26 FALL 2001

WITH THE DISCOVERYof the top quark at Fer-milab in 1995, the set of

six fundamental constituents of thestrongly interacting particles in theStandard Model was complete. Thelepton six-pack, however, still hadone gaping hole—the tau neutrino re-mained the most elusive of its stand-offish cousins. Experiments at SLACand CERN had determined that therewere at most three conventional,lightweight neutrinos, and there wasindirect evidence from the missingmomentum observed in decays of thetau lepton, but no definitive sight-ings (see “Pauli’s Ghost,” by MichaelRiordan, this issue).

Understanding why the tau neu-trino is so camera shy takes us backto the charged-current interaction.Electron neutrinos make electrons,

which interact almost immediatelyin detector material and produce adistinctive shower of particles. Muonneutrinos engender muons, whichtravel relatively undisturbed throughmatter before decaying and thusleave long tracks in detectors. Tauneutrinos make taus—and there’s therub. The tau lifetime is less than onethird of a trillionth of a second; mul-tiply this brief instant by the speedof light and you get a decay lengthliterally the size of a gnat’s whisker,about 90 microns. If a tau is ex-tremely relativistic, it may travel afew millimeters before decaying, buttracing these microscopic paths todiscover any kinks in them demandsextremely high-precision tracking.This requirement, coupled with thesmall likelihood of tau neutrinos everinteracting, makes detecting theman extraordinary challenge. To riseto that challenge came the DONUTexperiment, which ran at the Fermi-lab Tevatron in 1997.

There are two hurdles to over-come if you want to bag a tau neu-trino: first you gotta make ‘em, andthen you gotta take ‘em. To make tauneutrinos, you employ a variation onthe muon-neutrino recipe, but in-stead of using pions and kaons as theparents, you must employ muchheavier mesons. The lepton-numberconservation rule says you can onlymake a tau or its neutrino in tandemwith an antilepton of the third gen-eration. To get tau neutrinos, there-fore, you have to produce a parentparticle that includes a tau (whosemass is a whopping 1777 MeV, almosttwice the mass of a proton) in itsdecay chain. DONUT’s tau neutrinoscome from the decay of DS mesons,produced by slamming the Tevatron’s

IncomingNeutrino

OutgoingLepton

ChargedWeak Boson

Target

Calorimeter

Drift Chambers

Magnet

Emulsion Target

Steel Shield

15 meters

Muon Detector

NeutrinoBeam

A neutrino charged-current interaction.

DONUT detector for direct observation oftau neutrinos.

BEAM LINE 27

800 GeV protons into a tungsten tar-get. These mesons are also heavy(1970 MeV) and short-lived, so noattempt to focus them is made.

The aim of the experiment is tolook for charged-current scatteringof the resulting tau neutrinos. Thisprocess will make final-state taus,but how does one corral theseevanescent heavyweights? The keyto this experiment, the factor thatmakes it both beautiful and ratherdifficult, is the use of a large targetcomposed of photographic emulsionplates. These plates are just standardblack-and-white film medium, silverbromide, suspended in a gel. Insteadof a thin layer of this substance coat-ing a 35 mm piece of plastic film,however, DONUT’s plates containthe bulk emulsion deposited onsquare sheets about the size of a largepizza box (50 cm square). Chargedparticles traversing the emulsionleave tracks of exposed grains, andfinding a vertex (where the tau neu-trino interacted) that spawns akinked track (where the resulting taulepton subsequently decayed) pro-vides the “smoking gun,” evidencefor a tau neutrino.

Isolating such a vertex with atrack that kinks a scant few milli-meters downstream of it is no easyfeat. The emulsion stack is only 36meters downstream of the tungstentarget, which means that extensiveshielding must be used to keep theemulsion from being swamped bysuperfluous tracks. The usual pas-sive shielding (which filters outunwanted particles by letting theminteract in material) is not enough;DONUT’s emulsion stack needed ac-tive elements (magnetic fields todivert charged detritus) as well.

DONUT was also in the unusualposition of having to deal with neu-trino backgrounds, since the protoninteractions in the tungsten produceplenty of electron and muon neutri-nos, too. These extra neutrinos caninteract in the emulsion as well andrender the stack awash in tracks.

To help pick out a few specificbent needles from this huge haystackof tracks, the experimenters posi-tioned a series of scintillating-fibertracking detectors among the emul-sion plates. These trackers, togetherwith other detectors downstream ofthe emulsion array, helped identifythe particles and trace out theirpaths. The vertex and track recon-struction enabled a tiny, preciselyselected region of the emulsion to bepinpointed for closer scanning andmeasurement. The actual digitiza-tion and scanning of the emulsion,

One of the four tau neutrino eventsrecorded by the DONUT detector. Thetrack with a kink is a tau lepton decayinginto an invisible tau neutrino and anotherparticle.

28 FALL 2001

direct sightings of the tau neutrino,these events confirmed at last that itindeed exists.

AT ABOUT THE TIME thatthe DONUT collaborationwas looking for telltale

kinks, evidence was accumulatingfrom other quarters that the StandardModel might not be quite right. Sev-eral experiments had seen hints thattwo fundamental features thought tohold for neutrinos—the expectationthat they are massless and that theycannot change from one kind toanother—were in fact incorrect.Rather loose upper bounds had beenset on the neutrino masses by care-fully adding up all the visible energyin certain particle decays that pro-duce neutrinos. And the prohibitionon leptons jumping from one gen-eration to another had been probedby searches for exotic processes suchas µ → eγ.

One way to establish more strin-gent limits is to examine the curiouspossibility of neutrino oscillations(see box at left). Understandinghow such oscillations might oc-cur requires us to step into thesometimes-counterintuitive worldof quantum mechanics. According tothe Standard Model, neutrinos aremassless and conserve lepton num-ber. But if this is not in fact the case,then neutrinos can have differentmasses, and a neutrino created as onekind (say an electron neutrino,produced in the nuclear reactionsoccurring in the Sun’s core) couldevolve, or oscillate, into another kindas it zooms along through space.

Quantum mechanics tells us thatthe characteristics of any neutrinooscillation—its amplitude and repeat

section by tiny section, was done ata unique facility in Nagoya, Japan,one of the few of its kind in theworld. In July 2000, after data reduc-tion and a careful analysis to screenout backgrounds, the DONUT teamshowed the world four events with adistinctive kink in one track, indi-cating a tau lepton decay. The first

Neutrino Oscillations

HERE’S A GOOD WAY to think about neutrino oscillations. Sup-pose you could only hear a single pitch (or frequency) of sound at atime: That is, your “sound detector” could only tell that a note was a G,

or a B, or a D; it could not detect a mixture of these notes. If a note (you hear)starts out as a G (or a B or D for that matter), it would stay that way forever.This is the way that charged leptons behave.

Neutrinos play by different rules, however, and admit the possibility that anote originating as a G (an electronneutrino, say) can “detune” as it trav-els, developing B or D components (a muon or tau neutrino). This kind ofbehavior is analogous to that of thestrings in a guitar, which—onceplucked—can excite lesser vibrationson the other strings.

Does this mean that one wouldhear chords in neutrino beams? No—remember, you can detect only one“pitch” at a time. What it does mean,however, is that if you create a purebeam of 1000 Gs, say, and set up alistening post some distance away,you might get 990 Gs and 10 Bs. What fluctuates in neutrino oscilla-tions is the probability that a neutrinocreated as one specific kind will bedetected as that same kind (oranother kind) of neutrino.

All GNo B

Distance Traveled

Pro

babi

lity

Mostly GSome B

G

B

1

0

Oscillation probability: even though theneutrino beam starts out as 100 per-cent G’s, as it moves through space,the beam can gain (and lose, and thenregain) some B’s. What oscillates is theprobability.

BEAM LINE 29

length—are governed by four factors.Two of them are experiment-specific:the neutrino energy E and the source-to-detector distance L. The other twoare intrinsic properties of how theneutrinos fluctuate back and forthfrom one kind to another: the oscil-lation strength itself (the amplitudeof the wave) and the absolute differ-ence between the squares of theirtwo masses ∆m2 = |m1

2 − m22|. One of

the current problems in neutrino-os-cillation physics has to do with thislast parameter, ∆m2. Since there arethree known kinds of neutrino, therecan only be two independent differ-ences between their mass states. Thethree classes of experiments cur-rently yielding positive indicationsof neutrino oscillations—thosestudying neutrinos from the Sun,those observing neutrinos frommuons produced in Earth’s atmos-phere, and two accelerator-basedexperiments—do not yield a coher-ent set of ∆m2 values (see article byBoris Kayser, this issue). This dis-crepancy could point to a new, fourthkind of neutrino (called a “sterile”neutrino because it cannot interactwith ordinary matter). Or it couldimply that one (or more) of theseexperiments is not really seeingoscillations but another effect.

The one accelerator-based exper-iment that reported a neutrino-oscillation signal is the Liquid Scin-tillator Neutrino Detector (LSND),which detected neutrinos from pionsproduced by a medium-energy pro-ton beam at the Los Alamos NationalLaboratory. The collaboration ob-served an excess of events abovebackground that can be explained bythe oscillation of muon neutrinosinto electron neutrinos, with a ∆m2

of about 1–2 eV2. A result of no smallimportance, it cries out for indepen-dent confirmation. Enter the secondof Fermilab’s lissome leviathans, theBooster Neutrino Experiment.

BooNE’s main raison d’etre is totest the LSND result. It will eitherprovide confirming evidence (and getenough events to establish the νµ toνe transformation once and for all) orprove LSND wrong—and set morestringent limits on this particulartype of oscillation. There are somesurface similarities between LSNDand BooNE: both use large tanks filledwith mineral oil and lined withphotomultiplier tubes. But the detec-tor geometries are different (LSND iscylindrical, while BooNE is spheri-cal), the source-to-detector distanceL and beam energy E are different(although the ratio L/E for the two

Some of the 1200 photomultiplier tubeson the inside of the BooNE detector.

30 FALL 2001

The BooNE detector is a 12 mdiameter steel sphere filled with 770tons (about 250,000 gallons) of min-eral oil (see illustration on this page).A second light-tight inner walldivides the tank into an active cen-tral sphere and an outer “veto” shellused to flag exiting or extraneous in-coming particles. Charged particlesproduced by neutrino interactions inthe oil are detected by means of theblue Cerenkov light they generate,which will be detected by 1280 photo-multiplier tubes lining the inner sur-face of the central sphere or the 330tubes in the veto shell. Again, thecharged-current interaction willallow physicists to identify the kindof neutrino involved; a long muontrack (from the interaction of a νµin the mineral oil) will generate a pat-tern of Cerenkov light distinct fromthat produced by an electron shower(from a νe interaction).

As this article goes to press, con-struction of the BooNE detector isnearing completion, and the proton-extraction beam line and target-hornsystems are well underway. Datataking is expected to commence inearly 2002, after which the experi-ment should run for one to two years.If a signal is observed, a seconddetector can be built downstreamof the first—to make further detailedmeasurements of the parameters forνµ → νe oscillations. Such a resultwould add to the growing body ofevidence that neutrinos do indeedhave mass, and point to the excit-ing possibility of new physics beyondthe Standard Model.

NEUTRINO PHYSICShas long been a featuredperformer in Fermilab’s

experiments is the same), and thebackgrounds for BooNE will be dif-ferent from those of LSND.

The BooNE experiment will fol-low the standard recipe for making amuon neutrino beam, starting with8 GeV protons from the FermilabBooster, a low-energy synchrotron inthe Tevatron acceleration chain usedto boost protons from 400 MeV to 8GeV before passing them on to theMain Injector. Protons extractedfrom the Booster will strike a beryl-lium target; pions generated by thesecollisions will be focused by a mag-netic horn, and their decay will pro-duce the muon neutrinos speedingoff toward the detector.

Signal Region

Veto Region

Artist’s conception of the BooNEdetector, showing some of the photomul-tiplier tubes lining its 250,000-gallon tank.

BEAM LINE 31

multifaceted particle physics pro-gram. DONUT has made a major con-tribution, and the upcoming BooNEexperiment bids fair to continue thathistory. Future neutrino-beam op-portunities might include upgradingthe Booster to allow it to provideeven greater fluxes of neutrinos.There are also the exciting prospectsof “long-baseline” neutrino experi-ments where the distance betweenthe source and detector is hundredsof kilometers rather than hundredsof meters.

One such Bunyanesque experi-ment stretches all the way from Fer-milab to the Soudan mine in north-ern Minnesota, where the detectorfor the MINOS (Main Injector Neu-trino Oscillation Search) experimentwill be located. The MINOS beamstarts with 120 GeV protons from theMain Injector, which feeds the Fermi-lab Tevatron. The neutrino beam pro-duced by these protons hitting a tar-get will be directed north and slightlydownward, to intercept a six-kilotonsteel-and-scintillator detector in aniron mine some 750 km away—making MINOS one of the largestneutrino experiments on the planet.When this football field-length detec-tor is complete, it will capture neu-trinos beamed from Fermilab andfurther probe the mysteries of neu-trino oscillations. The combinationof neutrino energies and the longbaseline between source and detec-tor make MINOS sensitive to valuesof ∆m2 a hundred times smaller thanBooNE. This is the same range ofvalues that has been observed foroscillations of neutrinos in the up-per atmosphere (see article by JohnLearned in the Winter 1999 BeamLine, Vol. 29, No. 3).

Peering still further into thefuture, one can glimpse the in-triguing possibility of a “neutrinofactory”—a storage ring for muonswhose decay would provide a copi-ous source of muon and electron neu-trinos. Such a high-energy source ofelectron neutrinos would open wholenew vistas for further explorationof the weak interaction. But what-ever scenarios the future contains,neutrinos will surely play leadingroles in them.

Members of the MINOS collaboration be-fore one of its enormous steel plates.

For Further Information

DONUT home page:http://www-donut.fnal.gov

BooNE home page:http://www-boone.fnal.gov/

Juha Peltoniemi’s “ultimateneutrino” page:

http://cupp.oulu.fi/neutrino/

58 FALL 2001

PAUL NIENABER has the greatgood fortune to be part of twoextraordinary neutrino groups atFermilab. He joined the NuTeV col-laboration in 1992 and is now work-ing on data analysis from thatexperiment; in addition, he becamea member of the BooNE collaborationin 2000 and is participating in con-struction and testing of its detector.

The photograph above shows Paulinside the BooNE tank, surroundedby the photomultiplier tubes hehelped install. He is a Jesuit priest, aGuest Scientist at Fermilab, and amember of the physics faculty at theCollege of the Holy Cross in Worces-ter, Massachusetts, where he thor-oughly enjoys infecting college stu-dents’ minds with the manifoldpleasures of doing physics. This is hisfirst article for the Beam Line.

JOSHUA KLEIN received hisPh.D. from Princeton University in1994. He then moved to the Univer-sity of Pennsylvania where he hasworked on building and makingmeasurements with the SudburyNeutrino Observatory. He is cur-rently Research Assistant Professorof Physics at Pennsylvania.

SNO is in many ways the quin-tessential particle astrophysicsexperiment featuring the Sun as aneutrino laboratory and neutrinos assolar probes. Klein’s interest in SNOencompasses both of these features,including an interest in neutrinosthemselves as the Standard Model’smost enigmatic particle.

MICHAEL RIORDAN is Assis-tant to the Director at the StanfordLinear Accelerator Center, Lecturerin the History and Philosophy ofScience Program at Stanford, andAdjunct Professor of Physics at theUniversity of California, Santa Cruz.A Contributing Editor of the BeamLine, he is author of The Huntingof the Quark, and coauthor of TheShadows of Creation: Dark Matterand the Structure of the Universeand Crystal Fire: The Birth of theInformation Age. He leads a group ofhistorians and physicists researchingand writing the history of the Super-conducting Super Collider. In 1999 hereceived a Guggenheim Fellowshipto pursue research on this subjectat the Smithsonian Institution inWashington, DC. While there, he alsogot married again.

CONTRIBUTORS